The invention relates to an improved single vision spectacle lens.
Single vision lenses are prescribed when the patient is either farsighted or nearsighted and have the same focal power throughout (top to bottom).
Vision correction for myopia (nearsightedness) and hypermetropia (farsightedness) can be accomplished using spectacle lenses having appropriate spherical curves on the anterior (outer or front surface) and posterior (inner or back surface on the eye side) surfaces. Astigmatism can also be corrected when using toroidal or spherocylindrical surfaces.
Wearers of single vision lenses who are non-presbyopic have adequate accommodation to be able to bring to focus near objects when provided with the required correction of refractive error. However, the retinal images of near objects are too small in size to provide adequate visual comfort or contrast sensitivity especially at low or controlled illumination conditions, for example when reading a menu in a restaurant, or threading a needle indoors. The size of the focused retinal image is usually expressed in the form of angular magnification or paraxial magnification, in which magnification is defined as the ratio of the retinal image size formed by a particular lens to the image size formed by an emmetropic eye, i.e., an eye requiring no refractive correction. Image magnification may also be defined for one lens configuration relative to another, for example comparing two lenses with different base curves but providing the same spherical correction. Image magnification depends on the magnitude of the spherical for spherical equivalent) correction provided, as shown in equation 1.M=1/[1−t·F1/n]*1/[1−d·Fv]  Equation 1WhereinM is the lens magnification;t is the lens thickness;n is the refractive index of the lens material;F1 is the curvature of the front surface of the lens;d is the distance from the back vertex, or distance from the point of intersection on the lens of the principal axis, to the entrance pupil of the eye; andFv is the back vertex power, or the reciprocal of the distance, in air, from the back surface of the lens to the secondary focal point.
Equation 1 shows that image magnification is higher for plus powers and increases with plus power of a lens.
Equation 1 may be rewritten asSM=1/[1−(t/n)D1]*1/[1−d·D],  Equation 2WhereSM is the spectacle magnification, defined as the ratio of retinal image size when wearing a lens of power D to the image size in an emmetropic eye;D1 is the base curveD is the power of the lens in diopters.
Wearers of single vision lenses, in particular myopes who wear minus power single vision lenses may desire to enhance image magnification for near objects. In prior art single vision lenses, it is not possible to alter image magnification without changing refractive correction. Since single vision lens wearers require single vision lenses of a particular power in order to avoid image blur, they can not be provided a higher level of image magnification without causing them to have blurry vision.
Typically, non-presbyopic wearers of all types of vision correction, including users of single vision spectacle lenses or contact lenses experience reduced contrast sensitivity and visual comfort when viewing near objects, since near vision tasks typically require finer resolution and ability to function in low or controlled illumination environments, e.g., indoors at night.